Passive tr tubes

ABSTRACT

Passive TR tubes are disclosed using radioactive material to supply free electrons in the output cone-gap. The radioactive material is so located that it will not be damaged by the RF discharge in the gap, and can be included in the fabrication after the final brazing. Moreover, the electrical RF characteristics of the TR cell are maintained substantially or entirely without perturbation.

United States Patent [191 Durkee PASSIVE TR TUBES [75] Inventor: Earle Frederick Durkee, Danvers,

Mass.

[73] Assignee: Microwave Associates Inc.,

Burlington, Mass.

22 Filed: Sept. 1, 1971 21 Appl. No.1 176,982

[52] US. Cl. 333/13, 313/54 [51] Int. Cl. 1101p l/14 [58] Field of Search 313/54; 333/13 [56} References Cited UNITED STATES PATENTS 2,582,205 1/1952 Longacre 313/54 FOREIGN PATENTS OR APPLICATIONS 853,192 11/1960 Great Britain 313/54 Dec. 25, 1973 Primary Examiner-Roy Lake Assistant ExaminerDarwin R. Hostetter AttorneyAlfred H. Rosen and Frank A. Steinhilper [57] ABSTRACT Passive TR tubes are disclosed using radioactive material to supply free electrons in the output cone-gap. The radioactive material is so located that it will not be damaged by the RF discharge in the gap, and can be included in the fabrication after the final brazing. Moreover, the electrical RF characteristics of the TR cell are maintained substantially or entirely without perturbation.

10 Claims, 10 Drawing Figures PATENTEDnmzs 191s SHEET 10F 2 F/G. 4 V

EARLE FREDERICK DURKEE BY EQSEN & STEINHlLEER ATTORNEYS PASSIVE TR TUBES BACKGROUND OF THE INVENTION This invention relates to electric discharge devices of the kind used for receiver protection in high power radar, such as TR tubes or cells. As is known, a TR cell consists basically of a pair of electrodes surrounded by ionizable gas at low pressure and associated with some sort of resonant circuit, such as a tuned aperture in a waveguide, or a cavity resonator. The cell is usually lo cated across a transmission system and the arrangement is such that when the transmission-line energy at the radio frequency (RF) to which the resonant system is tuned exceeds a predetermined value a gas discharge at that RF takes place between the electrodes with the result that the cell acts in effect as an RF short across the line.

For complete receiver protection in high power radar, the TR tube must fire at the beginning of the first transmitter (e.g: magnetron) pulse. To initiate this discharge, it is necessary to have a number of free electrons in the gap between the electrodes, or the voltage must be raised to a level high enough to cause field emission from the cathode. This latter step has undesirable effects on the gas contained in the cell. Normally there are free electrons present in a gas volume. These are released photoelectrically or by high-energy cosmic or 'y-ray particles. A TR tube, however, is usually enclosed in a light-tight metal container and is surrounded by fairly massive pieces of metal, therefore the probability of ionization by external radiation is very small. For this reason, since the early days of microwave radar this supply of free electrons was maintained by a high voltage d-c discharge which was initiated before applying high voltage to the magnetron. The d-c electrode used for this purpose was called a Keep- Alive.

The TR tube will not, in general, protect receiver crystals if the keep-alive discharge is off; and, in particular, the very first pulse of leakage energy when the transmitter is turned on will be extremely large. Thus, rapid and reliable firing of the keep-alive under all circumstances' had to be ensured. This has been accomplished by producing a small amount of ionization within the tube by means of a radioactive substance. Without some radioactive priming several minutes would elapse between the application of keep-alive voltage and the actual d-c discharge. The material used has been primarily cobalt chloride producing 13 and y-rays; however, radium bromide which produces a, B and y-rays, and other materials have been used. The amount of cobalt chloride used had an equivalent radio-activity of 0.1 ng of radium. This amount of radioactivity is sufficient to guarantee the starting of the keep-alive d-c discharge within less than seconds after the application of voltage. For a more detailed discussion of this phase of the prior art, reference is made to Microwave Duplexers, Rad. Lab. Vol 14, McGraw-I-Iill 1946, Section 5.22, pages 216-217.

GENERAL NATURE OF THE INVENTION The Passive TR Tube has no keep-alive electrode and therefore no d-c discharge to supply free electrons in the cone-gap. In accordance with the present invention, radioactive priming is applied directly into the cone-gap. In order to be as effective as the d-c discharge it must cause an RF discharge within 0.01 microseconds after the beginning of the transmission pulse. (See Microwave Duplexers (ibid), Section 5.4 The Spike, pages 153-5). The reaction required here is 5 X 10 faster than the 5 seconds needed for initiating the keep-alive d-c discharge.

In terms of free electrons, one may consider figures as follows:

1. The normal d-c keep-alive discharge at microamperes produces 5 X 10 electrons per nanosecond 10" sec.). The discharge is decoupled for the RF circuit (cone gap) such that interaction (increase in power loss) is about 1 percent. The electrons in the gap are then 5 X 10 pen nanosecond.

2. Ignoring the effects of secondary electrons, a curie source produces 3.7 X l0' beta particles (electrons) per second, or 37 per nanosecond. One radioactive source for a passive TR tube (described below) is 1 curie per square inch and is 0.0156 square inches (1/16 X A) or a total of 15.6 millicuries. It will produce 0.58 electrons per nanosecond.

The ratio between the foregoing comparisons 1 and (2) is 10 This analysis indicates that such a radioactive beta particle source is far from being equivalent to the prior art d-c keep-alive discharge in supplying free electrons in the cone-gap. These figures may be in error by a few orders of magnitude, particularly the decoupling factor between the d-c discharge and the conegap. However, this analysis does indicate a need for more free electrons in the cone-gap to produce results equivalent to the TR tube with a keep-alive DC discharge.

As is well-known, alpha particles, while not very penetrating, will produce vastly more ion pairs in gas than will beta particles. Thus, alpha particles which are emitted from polonium or uranium at a velocity of 5 percent of the velocity of light, will penetrate air to a range of only 1.7 cm at MEV, but will produce 4,000 ion pairs per mm. Beta particles, on the other hand, are high speed electrons which travel at a speed nearly that of light, and can penetrate aluminum to a depth of 0.040 inch. A 3 MEV beta particle has a range of 13 meters in air but produces only 4 ion pairs per mm. Gamma rays travel at the speed of light and have much higher penetration than alpha or beta particles, but produce no ions directly in travel through various materials. They produce ionization of surrounding gas indirectly, by secondary electrons knocked out of materials penetrated, such as the copper walls of a TR tube. Cobalt 60, which has been used in prior art (active) TR tubes, produces mainly gamma rays. Beta particles also produce secondary electrons in materials penetrated.

In active-priming TR tubes beta and gamma producing materials have been used in preference to alpha emitters because in order to preserve the radioactive material for a long period of time it was important to keep it away from the discharge in the gap, that is, to locate it in another part of the tube somewhat remote from the discharge gap. Typical locations are on one of the iris vanes. This consideration required a radioactive source emitting a radiation (i.e: beta or gamma) capable of penetrating the gap electrode material and producing secondary electrons in the vicinity of the gap, preferably in the gap itself. Alpha particles which travel in a straight line and will not significantly penetrate metal (a sheet of paper will stop them) are not suitable for this application. Yet the passive TR tubes to which the invention is addressed present the same considerations to a much greater degree (approximately 10 as is developed above).

It is therefore the primary object of this invention to provide passive TR tubes employing radioactive keep alive materials in new ways to improve their effectiveness. Thus, according to the general principle of the invention radioactive material is located beside the gap spaced from the path of the discharge which occurs across the gap during firing of the TR-tube in the presence of transmitter energy. In one exemplary embodiment of the invention, the radioactive material is supported on a rod or the like which extends between the narrow walls of the rectangularwaveguide forming the enclosure, the radioactive material being disposed to radiate its emission into the close vicinity of the gap and into the gap itself. In some embodiments, dielectric supporting members are employed to support the radioactive material, thus minimizing perturbation of the RF electrical properties of the TR tube. In all embodiments of the invention, the structure permits the radioactive material to be put in place after the tube has been brazed.

According to the invention, alpha particle emitters may be used to advantage, and materials that emit beta particles and/or gamma rays may be used more effectively, and perturbations of other properties of TR tubes (e.g: recovery or gas deionization time), as well as of the RF electric field, are held to a minimum, if not avoided entirely.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION Exemplary embodiments of the invention are described with reference to the accompanying drawings, in which:

FIG. 1 is an isometric view, partly broken away, of a passive TR tube according to the invention;

FIG. 2 is a partial longitudinal section of FIG. 1 taken between the top and bottom wide walls of the waveguide;

FIG. 3 is a section on line 3-3 of FIG. 2;

FIG. 4 is a section on line 44 of FIG. 2;

FIG. 5 is an enlarged view of a portion of FIG. 2 contained in the dashed rectangle 40;

FIG. 6 is a further-enlarged partial section taken on line 66 of FIG. 5;

FIG. 7 is a cross-sectional view of another embodiment of the invention;

FIG. 8 is a cross-sectional view of a third embodiment of the invention;

FIG. 9 is a section along line 9-9 of Flg. 8; and

FIG. 10 is a section along line 10-10 of FIG. 8.

FIG. 1 illustrates the general arrangement of the invention. An electric discharge device, here illustrated as a TR cell, comprises a section 10 of a rectangular waveguide sealed off at the ends by glazed slots 11 and 12 and containing input and output gap assemblies 13 and 14. The spacing, along the waveguide axis, between the two gap assemblies, and between each such assembly and the adjacent glazed slot is, typically, equal to a quarter guide-wavelength of the center frequency of the operative frequency band. The waveguide section is sealed and filled with ionizable gas (not shown) at low pressure. For description purposes, assume that the waveguide 10 has its broader walls 15 and I6 horizontal-these being the top wall and bottom wall, respectively. The remaining two walls 17 and one that is not shown, are side walls or narrow walls.

The input gap assembly 13 typically comprises two cone-shaped electrodes 21 and 22 having bases affixed to the top and bottom walls, respectively, and extending from their respective bases toward each other to confronting tips having a discharge gap 19 between them. The gap assembly is fitted with two iris members 18 adjacent one narrow wall 17 and another (not shown) adjacent the other narrow wall (also not shown), in a well-known configuration, to provide a tuned aperture across which the gap 19 is connected.

The output gap assembly 14 similarly comprises two electrodes 30 and 32, and iris members 31 and another that is not shown, with a gap 35 between the electrodes across the tuned aperture thereby provided. This much of the structure illustrated in FIG. 1 is old, and wellknown.

According to the present invention, a rod 36 is located in the waveguide 10, passing from one narrow wall to the other. The rod carries a radioactive material (not shown in FIG. 1) on an intermediate portion 37 of the rod which is closely beside the gap 35. Conveniently, the rod passes through the sidewalls, as will be explained.

FIG. 2 is a longitudinal sectional view of FIG. 1, taken in a plane parallel to the top and bottom walls, just above the rod 36, cutting through the tip of the upper electrode 32, and showing the two iris members 31 and 33 of the output gap assembly, as well as both narrow walls 17 and 34. The portion 37 of the rod 36 for the radioactive material (not shown in FIG. 2) is flattened, as by being milled out. The dashed-line rectangle 40 encloses a portion of FIG. 2, the contents of which are shown enlarged in FIGS. 5 and 6.

FIG. 3 is a section along line 33 of FIG. 2. FIG. 4 is a section along line 4-4 of FIG. 2. The radioactive material 44 is located in a flattened surface 38 of the intermediate section 37 of the rod 36. As will presently appear, the radio-active material itself has a surface which emits radiation and that surface is oriented broadside toward the gap 35. A second rod carrying radioactive material can, if desired, be included on the other side of the gap 35 in a location that is indicated in dotted line at 45.

Referring now to FIGS. 5 and 6, the enlarged parts within the box 40 in FIG. 2 include the radioactive material 44 having a surface 46 emitting radiation capable directly or indirectly of ionizing a gas of the kind usually enclosed in TR cells. The radioactive material 44 exemplified in FIG. l-6 is a foil of stainless steel, on which titanium has been sputtered, and then made radioactive by radioactive tritium, i.e.: a tritiated foil". The foil is spot-welded to the rod 36, which may be made of stainless steel. In a typical X-Band TR cell, the rod may be one-sixteenth inch in diameter, the foil onefourth inch long by one-sixteenth inch wide, and the distance from the radiating surface 46 of the foil to the gap 35 should be less than approximately one-eight inch. Samples of X-Band and Ku-Band TR Cells made according to the invention have employed a spacing distance between the surface 46 and the center-line of the gap 35 which was 0.040 inch in one instance, and a distance between the center-line of the rod 36 and the center-line of the gap 35 which was 0.080 inch in another instance. The length of the gap 35 is typically about 0.0050 inch. This technique makes it possible to locate the radioactive material closely beside the discharge gap, i.e.: within about 0.050 inch of the gap, without putting the radioactive material in the gap itself, and without disturbing the electrical (e.g.: bandpass, V.S.W.R. phase shift, insertion loss) RF characteristics of the TR Cell.

A principal problem in constructing a passive TR Tube employing radioactive keep-alive or priming is to find a way to place the radioactive material in the tube without ruining it during the braze. It is clear that even if this can be done, it must be done with an installed structure which does not affect the RF properties of the tube. The present invention satisfies all of these requirements. Because the described tritiated foil source of ionizing radiation will be damaged if its temperature is raised above 130C, the foil 44 is not suitable for location within the gap 35, where the are occurring when the gap is fired will heat the ionized gas between the electrodes to a temperature higher than 130C. Hence, according to the present invention, the foil 44 is located beside the gap 35, spaced from the path of the discharge, and the radiating surface 46 is oriented to face the gap, and thereby to direct its radiation primarily toward the tips of the electrodes 30, 32 and into the gap itself. If the tube is to be hard-brazed at a high temperature that would damage the radioactive source, the rod 36 bearing the radioactive material can be made separately, and installed after brazing, as will presently be explained. These features are available for any type or kind of radio-active source.

Prior attempts to use radioactive aids to ionization of the gas in a TR tube have, apparently, avoided locating the radioactive material close to the discharge gap, and as a result the development of passive TR cells has been hindered. Although the above-referenced Microwave Duplexers mentions on page 217 the practice of putting a drop of cobalt chloride solution on the cone adjacent to the keep-alive electrode before sealing off the tube, this practice subjects the radioactive material to the action of the RF discharge and to the brazing temperature and so, while it was apparently tried more than twenty years ago, it has not provided a technique that is useful in passive TR tubes. In one early construction, radioactive cobalt chloride has been located on a surface of one of the iris members 31 or 33, with its radiating surface facing in the axial direction of the waveguide, away from the discharge gap. This was used in TR cells having DC keep-alive electrodes, and was also found to be ineffective as a keep-alive structure by itself. Another technique, which has been proposed for use without a d-c keep-live electrode, involves locating on the confronting edges of the iris members circular discs bearing radioactive material facing the gap. This technique is defective for several reasons: the discs alter the RF electrical properties of the TR tube, they cannot be installed in a hard-brazed cell after brazing, and they are too far from the gap to be effective. A similar technique, which locates the radioactive material on a support extending into the waveguide from a sidewall some distance away from the gap assembly is deficient for the same reason; while moving the radioactive material closer to the side wall in this construction reduces the adverse effects on the RF properties, it also increases the distance from the discharge gap. An attempt to locate the radioactive material on metal posts or rods extending between the broad walls of the waveguide adjacent the confronting edges of the iris members suffered similar deficiencies of distance from the discharge gap. Moreover, this location is near highvoltage regions in the broad walls, referred to the dominant waveguide mode of transmission, and thus the contact requirements between the rods and the wall are more rigorous. Further, metal parts in this location change the RF characteristics of the TR cell, and therefore they must be in place when the tube is tuned; again, not a suitable arrangement for hard-brazed TR tubes.

The choice of radioactive material is not limited to the tritiated foil which is described herein. An alpha emitter, or a source of gamma rays, may also be used, as well as other sources of beta emission. Moreover, isotopes which emit two or more of these radiations may be used. For example, Pb decays with a 21-year half-life by beta emission of Bi which in turn ejects a second beta ray with a 5-day half-life to become P0 an alpha emitter decaying with a 140-day halflife to stable Pb The relationship between these various half-lives is such that the products will come into secular equilibrium within a year after the lead is electrodeposited on the surface that will hold it. Thus it will provide an alpha source of 5.4 MeV radiation, with a half-life of 21 years, growing in at such a rate that it will have its ultimate strength 5 months after deposition. This source will include 1. 16 MeV beta radiation from Hi yielding 1 beta particlefor each alphaparticle after equilibrium is established, bu t this ratio can be altered to a lower beta proportion by depositing an appropriately thin layer of Pb. The use of compound emitters of this type is within the scope of the present invention.

Passive TR cells according to the invention can be produced commercially, that is, the design is reproducible with a yield of good TR cells that can be sold competitively with TR cells having a prior art d-c keep-alive electrode structure. The following characteristics have been observed for TR cells operated at X-Band or Ku- Band (a) with the standard DC Keep-Alive electrode (b) without either a Keep-Alive electrode or a radioactive material to provide ionization of the included gas, and (c) as passive TRs with tritiated foil radioactive source material according to FIGS. 1-6:

(pulse) l to l0 Watts 1 to 10 KW Watts peak Flat Leakage (pulse) 50 to 100 mW 50 to 100 mW 50 to 100 mW Breakdown (CW) I00 10 500 mW l to lo KW lto l0 Watts The rod 36 may be made of a metal; stainless steel is mentioned above as one example of a useful metal. When a metal or other electrical conductor is used, location of the rod as shown in FIGS. l-6 avoids disturbing the electrical characteristics of the TR cell to any objectionable extent. A metal rod in this position has very little effect on VSWR or loss. It is in actuality a shunt capacitance which is negligible compared to that of the electrodes 30, 32. The rod can be installed, after the TR cell, without gas or the rod, has been assembled and brazed. Thus, if two holes one-sixteenth inch in diameter are provided in the side walls 17 and 34, the rod can be inserted through them, the radioactive material can be located and oriented relative to the gap 35, and

then the rod can be welded to the side walls. This is of particular advantage in hard-brazed TR cells, which are in demand for their long life. While the radioactive material will not withstand the temperatures used in hard brazing (e.g.: 650850C.), and in fact the temperature employed for welding the rod to the side walls is even higher (e.g.: 1 ,OOOC.), the welding heat is applied for only one-half to one second in an inert-gasshielded area and is largely insulated from the radioactive material by the relatively great lengths of the rod separating it from the side walls and the relatively large heat sink constituted by the side walls.

Insertion of a gas, and provision of tubulations for that purpose, may be done in any known manner. Indeed, the rod 36 could be inserted through copper tubulations, which could then be pinched off, in the usual manner, to clamp it in place and to seal the cell. FIG. 7, which is a cross-section of a device similar to FIG. 1 looking toward the output end from a point between the gaps 19 and 35, shows a hollow rod, or tube 36', welded to the sidewalls 17 and 34 at 51 and 52, respectively. The tube has a bore 54 in its sidewall within the waveguide 10, through which an ionizable gas may be introduced into the TR cell after all other assembly steps have been completed. One end of the tube can be sealed before the gas is inserted, or both ends can be sealed at the same time, as desired. Also, the tube 36 can be a rod which is axially apertured (e.g.: drilled out) for only part of its length, rather than for its full length as shown.

The concept of locating an electrically conductive element in a TR cell in the manner shown in FIGS. 1-7, such that its presence does not appreciably affect the RF electric field in the TR cell, is in itself not new, being shown, for example in British Patent No. 748,885. However, as is noted above, prior attempts to use radioactive aids to ionization of the gas in the cell have avoided locating the radioactive material close to the discharge gap, and as a result the development of passive TR cells has been hindered.

The rod 36 or tube 36' need not be made of a metal or electrical conductor. Dielectric materials may be used, for example, glass, ceramics, and sapphire. A dielectric which is in practical effect transparent to the waves intended to be propagated in the waveguide section 10 is preferred. A support for the radioactive material which is made of such a dielectric can be located more freely within the waveguide. Thus, as is shown in FIGS. 8, 9 and 10, a dielectric support 61 bearing the radioactive material 44 can be mounted in an aperture in a wide wall 15, extending downward to a location beside the gap 35. Moreover, if desired, a dielectric support may extend between two opposite walls of the waveguide, like the metal rod 36. The radio-active material can be mounted to the dielectric rod by any suitable cement (e.g.: epoxy cement). The rod is preferably supported in the wall by a metallizing-soldering technique, as is well-known.

I claim:

1. In an electric discharge device for use in high frequency electromagnetic wave systems, said device having discharge electrode means providing a gap for forming in said gap an electric discharge by electromagnetic waves, but lacking any means to establish a d-c discharge to supply free electrons in said gap, an ionizable gas surrounding said electrode means to facilitate the formation of said discharge and an envelope surrounding said electrode means for containing said gas, the improvement comprising a radioactive material having a surface emitting radiation capable of ionizing said gas and means within said envelope supporting said material beside said gap in the path of electromagnetic waves propagating in said envelope and spaced from the path of said discharge a distance which is not substantially greater than ten times the length of said gap, said material being oriented to present said surface broadside to said discharge, whereby to radiate toward said gap and thereby to provide an effective entirely passive keep-alive for said electrode means.

2. Discharge device according to claim 1 in which said surface is flat.

3. Discharge device according to claim 1 in which said supporting means is an elongated element affixed to said envelope and carrying said radioactive material, and is structured to minimize perturbations of the electrical properties of said device in the absence of said supporting means.

4. Discharge device according to claim 1 in which said envelope comprises a rectangular waveguide and said gap is formed between two electrodes extending toward each other between the broadwalls of said waveguide.

5. Discharge device according to claim 4 in which said supporting means is an elongated member extend ing from at least one wall of said waveguide to said gap, the material of said elongated member being chosen from electrical conductors and dielectrics.

6. Discharge device according to claim 5 in which said elongated member extends between the narrow walls of said waveguide and is affixed to both of said narrow walls.

7. Discharge device according to claim 5 in which said elongated member is a hollow tube having an end passing through said wall and a bore is provided in the sidewall of said tube within said waveguide, whereby said elongated member may be used also as an exhaust tubulation.

8. Discharge device according to claim 4 in which said supporting means is an elongated member of dielectric material extending from at least one broad wall of said waveguide to said gap.

9. Discharge device according to claim 1 in which said radioactive material is a metal foil.

10. Discharge device according to claim 1 in which said radioactive material is capable of emitting two forms of radiation, one of which is capable of ionizing said gas directly, and one of which is capable of ionizing said gas indirectly by producing secondary electrons from said electrode means. 

1. In an electric discharge device for use in high frequency electromagnetic wave systems, said device having discharge electrode means providing a gap for forming in said gap an electric discharge by electromagnetic waves, but lacking any means to establish a d-c discharge to supply free electrons in said gap, an ionizable gas surrounding said electrode means to facilitate the formation of said discharge and an envelope surrounding said electrode means for containing said gas, the improvement comprising a radioactive material having a surface emitting radiation capable of ionizing said gas and means within said envelope supporting said material beside said gap in the path of electromagnetic waves propagating in said envelope and spaced from the path of said discharge a distance which is not substantially greater than ten times the length of said gap, said material being oriented to present said surface broadside to said discharge, whereby to radiate toward said gap and thereby to provide an effective entirely passive keep-alive for said electrode means.
 2. Discharge device according to claim 1 in which said surface is flat.
 3. Discharge device according to claim 1 in which said supporting means is an elongated element affixed to said envelope and carrying said radioactive material, and is structured to minimize perturbations of the electrical properties of said device in the absence of said supporting means.
 4. Discharge device according to claim 1 in which said envelope comprises a rectangular waveguide and said gap is formed between two electrodes extending toward each other between the broad walls of said waveguide.
 5. Discharge device according to claim 4 in which said supporting means is an elongated member extending from at least one wall of said waveguide to said gap, the material of said elongated member being chosen from electrical conductors and dielectrics.
 6. Discharge device according to claim 5 in which said elongated member extends between the narrow walls of said waveguide and is affixed to both of said narrow walls.
 7. Discharge device according to claim 5 in which said elongated member is a hollow tube having an end passing through said wall and a bore is provided in the sidewall of said tube within said waveguide, whereby said elongated member may be used also as an exhaust tubulation.
 8. Discharge device according to claim 4 in which said supporting means is an elongated member of dielectric material extending from at least one broad wall of said waveguide to said gap.
 9. Discharge device according to claim 1 in which said radioactive material is a metal foil.
 10. Discharge device according to claIm 1 in which said radioactive material is capable of emitting two forms of radiation, one of which is capable of ionizing said gas directly, and one of which is capable of ionizing said gas indirectly by producing secondary electrons from said electrode means. 